Phantom carrier circuits



April 27, 1926. 1,582,073

I J. T. O'LEARY PHANTOM CARRIER CIRCUITS Filed August "1 1925 mvENTol'z J2 QZemy BY I ATTORNEY Patented Apr. 27, 1926.

UNITED STATES PATENT orFicE.

.rosnrn 'r. oLEAizY, OF ELIZABETH, NEW JERSEY, assrenon 'ro AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A conronarron on NEW YoEK.

PHANTOM CARRIER CIRCUITS.

Application filed August 11, 1925. Serial No. 49,617.

cuits and more particularly to arrangements for employing carrier transmission upon phantom conductive systems.

It has heretofore been the practice in ordinary telephone transmission to phantom the telephone circuits so that each pair of conductors not only acts as a series conductive system or a side circuit but also acts as one side of a phantom circuit, the two conductors in parallel constituting one conductor of the phantom path while the two conductors of a similar side circuit operate in parallel as the return path for the phantom circuit. 'It has not, however, been the practice to utilize the phantom circults thus formed for carrier transmission ber cause the requirements with respect to'unbalance and crosstalk at carrier frequencies have been too great. The carrier frequency unbalance and crosstalk of the phantom are gradually being reduced, however, and there is now presented the problem of associating filters with each other so as to properly separate the side voice circuits and side carrier circuits from each other and at the same time separate the phantom voice circuit and phantom carrier circuit from each other and from the side circuit channels;

It is one of the objects of the present invention to provide a suitable arrangement of filters for effecting the necessary separa tion between the various types of circuits without introducing undesirable transmission losses and without producing any undesirable effects upon the filters due to the presence in the same network of other filters, t

These objects as well as other objects 01 the invention may be attained by means of,

the arrangements set forth in the following description and illustrated in the accompanying drawings, Figure 1 of which shows a circuit diagram em odying the invention in its preterred form and Figs. 2 and 3 at which show how the high pass and iuw pass filters function in the phantom cir cuits.

comprising a pair of conductors which may be phantomed in a well known manner so that each pair of conductors acting in parallel operates as one side of a phantom circuit. The phantom connections are established by means of transformers 10 and 10, from the middle points of whose windings taps 11 and 11 are led to form the phantom terminals of the system. Composite sets OX and CK are included in the lines HF L and HFL by means of which each of the four conductors may be employed for Morse telegraph transmission. The composite sets are merely shown schematically in the drawing, these sets being of a type well known in the art so that detailed illustration is unnecessary.

In order that the line HFL may be em- GL is provided which leads to carrier equip- Referring to the drawing, HFL and HFL represent two telephone lines, each ment of any type well known in the art,

and in this branch ahigh pass filter HF is included. This high pass filter is of the broad band type which freely transmits a band of frequencies within certain limits while sharply attenuating and suppressing thefrequencies lying without the limits of the band. The filter is so designed as to pass the various carrier frequencies employed in the system together with their associated side bands, but it will not permit ordinary voice currents transmitted over the line HFL to pass into the circuit GL The ordinary voice currents transmitted over the conductor HFL must be separated from the carrier frequencies and transmitted through the transformer 10 to the voice side circuit channel VL In order to accomplish this result, a lowpass filter LF is connected in the branch of the line circuit HFL leading'to the voice side circuit VL L The filter LF is of the broad band type and freely transmits all frequencies lying within the voice range while suppressing all frequencies above the voice range. The carrier frequenciesare thereby prevented from entering thevoice side circuit VL A In similar manner, the line HFL" 1s provided with We branches, one

carrier equipment.

through the high pass filter HF to the side carrier circuit CL, and the other leading through the low pass filter LF to the voice side circuit VL The arrangement so far described is that ordinarily employed for superposing a plurality of carrier channels on an ordinary voice channel. It is possible to phantom a pair of circuits of this type for ordinary voice transmission by means of the phantom coils 10 and 10 with their phantom conductors 11 and 11. This follows at once from the fact that the unbalance between the phantom circuit and the side circuits at voice frequencies is no greater with the arrangement above described than in the case of circuits which are not arr'z-inged for multiplex carrier transmission, and the voice currents transmitted over the phantom, in so far as any unbalance occurs between the two conductors of the side circuit. are prevented from entering the carrier circuit, such as the circuit 011,, by the high pass filter. The effect upon the phantom of the filter Ll, in the two conductors of the side circuit is to introduce impedance due to the series coils of the filter but this impedance is not serious at the ordinary voice frequencies. If, however, we attempt to transmit carrier frequencies over the phantom, the induetanceof the series elements of the filter LP. or LF is relatively much greater, so that not only must this problem be overcome but the problem of selection between the carrier phantom and the voice phantom must also be overcome.

In accordance with the present invention it is proposed to provide acarrier phantom connection by introducing phantom coils, such as 20 and 20, between the high pass filters HF and HF, and the side carrier terminals CL and GL Conductors 21 and 21 are led from the midpoints of the phantom coils, as indicated, to form the phantom carrier terminal CL, which leads to suitable The shunt elements of the high pass filters, being inductance elements and being bridged across neutral points of the two conductors ofthe side circuit with respect to the phantom, would not materially affect transmission of carrier frequencies over the phantom and since the series elements of the high pass i ters are apacities the impedance due to these series elements. will be relatively small since the impedance of a capacity is relatively low at high frequencies.

While the arrangements above outlined enable the transmission of both voice and carrier frequencies over the phantom circuits without introducing material transmission losses, there remains the problem of providing for frequency separation between the phantom voice channel and the phantom carrier channel. As will be seen, this problem is solved in accordance with the arrangements of the present invention in such a manner as not only to effect this separation by u'tilizii'ig the same filters that provide for frequency separation between the voice channel and carrier channel of the side circuits, but, in addition, the filters in the voice and carrier side circuit channels are so arranged that they introduce substantially no attenuation in the corresponding phantom channels. To accomplish these results, the low pass filters, such as LF and LR, are so constructed that the shunt elementof each filter section is formed of two equal capacities. Connections then extend from the midpoint of each shuntclement of'one filter, for en:- ample, LF to the corresponding n'iidpoint of the corresponding shunt element of the other filter, such as .l iF Similarly, the

high pass filters such as HF and HF. are

formed of sections, each of which comprises a series inductance. These inductanccs are likewise divided into two equal parts with the midpoiuls ot' each series inductance of one. high pass filter, such as Illconnected to the'inidpoint of the corresponding shunt inductance of the other high pass filter, such as HF \Vith the filters thus interconnect ed. the effect of a given low pass filter in a side circuit branch is to constitute, with the corresponding low pass filter in the associated side circuit branch, at similar low pass filter for the voice frequency phantom. Similarly, the two high pass filters HF and HF, constitute together a high pass filter of the same type for the corresponding carrier frequency phantom.

The effect of the two filters LF and LF, in forming a low pass filter for the voice frequency phantom VL, will be clear from Fig. 2, in which the circuits have been so diagrammed as to show the relation of the elements of the filter in the phantom circuit. If we consider, for example, the elements shown in the heavy line portions of thecircuit of Fig. it will be seen that the series inductances in the upper conductor of the upper side circuit, together with the upper halves of the capacities of the filter LF are directly connected to the upper capacities of the filter LF and the series inductances in the upper conductor of the lower side circuit to form a filter which is exactly like the filter LE... Now, obviously, since one half of the voice phantom current tlows in one direction through the upcr conductor of the upper side circuit and returns through the upper conductor of the lower side circuit, and since these conductors have included therein a filter which in all respects is equivalent to the low pass filter in the side circuit, the phantom circuit is limited to the samerange of frequencies as the side circuit, and, furthermore, the filter by its very nature subjects the voice frequency currents in portion of the phantom to substantially no attenuation. Similarly, it will be seenthat the lower half of the filter LF, and the lower half of the filter LF constitute together a similar low pass filter for the other two conductors of the phantom, that is, the lower conductor of the. upper side circuit and the lower conductor of the lower side circuit. tom current fiows through the last mentioned filter without substantial attenuation and the range of frequencies is limited in the same manner as alreadly described. Consequently, the two low pass filters in the side circuits constitute together a low pass filter for the voice frequency phantom and offer substantially no attenuation for Voice frequencies transmitted over the phantom.

Referring now to Fig. 3, which shows the relation of the high pass filters in the phantom circuit and noting the parts shown in heavy lines, it becomes at once evident that the upper half of the filter HF co-operates with. the upper half of the filter HF, to form a high pass filter exactly like the filter I-IF itself. Consequently, one half of the carrier frequency energy transmitted over the phantom flowing in one direction Over the upper conductor of the side circuit and returning over the corresponding upper conductor of the lower side circuit passes through a high pass filter which ofiers sub stantially no attenuation to frequencies in the carrier range and which also restricts the frequencies transmitted over this part of the phantom to the carrier range. Likewise,

it will be seen by noting the elements shown in the light lines of Fig. 3 that the lower half of the filter HF and the lower half of the filter I-IF constitute together a high pass filter exactly like the filter HF for the other two conductors of the phantom circuit.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely difij'crent from those illustrated without departing from the spirit and scope of the appended claims.

lVhat is claimed is:

1. In a transmission system, a pair of transmission lines adapted to transmit either ordinary voice frequency currents or carrier Frequency currents, a pair of hanches I for each line, one branch extending to a voice I frequency terminal and the other branch extending to carrier equipment, a phantom conductor connected to the first mentioned branch of each line to "form a phantom circuit for the transmission of voice frequency currents, a phantom conductor connected to the second mentioned branch of each. line to "form a phantom circuit for the transmission of carrier frequency currents, a high pass filter in the first mentioned branch of each line including a plurality of sections,

The other half of the phan each comprising a series impedance and a shunt impedance, connections from the mid points of the shunt impedances of one of said low pass filters to the midpoints of the corresponding shunt impedances ofthe other low pass filter, a high pass filter in the second mentioned branches of each line, each high pass filter includinga plurality of sections, each comprising a series impedance and a shunt impedance, and connections from the midpoints of the shunt impedances of one of said high pass filters to the midpoints of the corresponding shunt impedances of the other high pass filter.

2. In a transmission system, a pair of transmission lines adapted to transmit either 'voice frequency currents or carrier freconnected to the midpoints of each of said last mentioned phantom coils so that the two phantom conductors establish a carrier frequency phantom circuit, a low pass filter in each of said first mentioned branches, said low pass filter including a plurality of sections and each section comprising a series inductance in each line conductor and a divided capacity in shunt with the line conductors, a connection extending from the midpoints of the shunt capacities of each high pass filter to the midpoints of the corresponding shunt capacities of the other low pass filter, a high pass filter in the second mentioned branches of each line, each high pass filter including a plurality of sections and each section comprising a series capacity element in each line conductor and a shunt inductance, and a connection from the midpoints of the shunt .inductances of one high pass filter to the midpoints of the corre sponding shunt inductances of the other high pass filter. i

3. In a transmission system, a pair of transmission lines adapted to transmit either ordinary voice frequency currents or carrier frequencycurrents, a pair of branches for each line, one branch extending to a voice frequency terminal and the other branch extending to carrier equipment, a phanton'i conductor connected to the first mentioned branches of each line to form a phantom circuit for the transmission of voice frequencies, a phantom conductor connected to the second mentioned branches of each line to form a phantom circuit for the transmission of carrier frequencies, a low pass filter in each of said first mentioned branches and a high pass filter 111 each of Sand first rinentioned branches, said low pass filters being so balanced with respect to the voice frequency phantom and being so relatedto each other as to jointly comprise a low pass filter for the voice frequency phantom, and said high pass filters being so balanced with respect to the carrier frequency phantom and being so related to each other as to jointly comprise a high pass filter for the *arrier frequeney'phantom.

In testimony whereof, I have signed my name to this specification this 5th day of August, 1925. A I

JOSEPH T. OLEARY. 

